In recent years, attention is being attracted to wireless power supply that does not use power source codes and power transmitting cables as a power supply system that supplies to batteries mounted on, such as, hybrid cars or electrically-powered cars. A resonance type as one of this wireless power supply techniques is known (refer to PTL 1).
The wireless power supply adopting resonance type is configured with a power supply-side resonance circuit and a power receiving-side resonance circuit arranged apart to each other. The power supply-side resonance circuit and the power receiving-side resonance circuit are respectively configured with resonance coils and capacitors connected to the resonance coils.
A resonance frequency f of the power supply-side and the power receiving-side resonance circuit is represented by the following equation (1):f=1/(2πsqrt(LC))  (1)where L denotes the inductance of the resonance coil, and C denotes the capacitance of the capacitor.
By having the power supply-side resonance circuit and the power receiving-side resonance circuit to be resonant, electric supply is enabled from the power supply-side to the power receiving-side in a contactless manner.
However, in the aforementioned power supply system, the resonance frequency f possibly deviated from the target value due to the scatter in the capacitance C of the capacitor and the inductance L of the resonance coil which occurs when manufacturing the products. Such a deviation from the target value in the resonance frequency f may have caused a decrease in transmitting efficiency. There is shown in FIG. 4 the transmitting efficiency of the power supply system observed when a variation ranging from 0% to ±10% occurs in the capacitance C of the capacitor.
As shown in the figure, when the capacitance C of the capacitor is at the target capacitance, the transmitting efficiency can be approximately 97.8%. However, as the difference between the capacitance C of the capacitor and the target capacitance is increased, the transmitting efficiency is decreased and when an error of −10% is produced, the transmitting efficiency is decreased to as low as 94.3%.
Then, it can be considered that a turn number of the resonance coil is made adjusted so as to target the resonance frequency f even if the variation is produced in the capacitance C of the capacitor. However, the excessive length is required in the resonance coil in order to make the adjustment of the turn number possible. In a state where the excessive length can be arbitrarily deformed, a floating inductance may be undesirably produced in accordance with the shape of the excessive length portion. Thus, even if the turn number is made adjusted, the shape of the excessive length portion is deformed afterward, and the floating inductance or a variation in the floating inductance may cause a repeated deviation from the target value in the resonance frequency f. Such a problem as this arises.